Abstract

Prolonged droughts are predicted for some parts of the Amazon; however, it is still unclear how Amazonian trees will respond to water stress under the ongoing increase in CO2 concentration. The aim of this study was to assess the effect of elevated CO2 (eCO2) and drought on photosynthetic rates, water-use efficiency, and biomass allocation in andiroba (Carapa surinamensis). The plants were grown in pots at ambient (400 ppm CO2) and eCO2 (700 ppm) at two water regimes, soil at 50% field capacity, FC (drought) and soil at 100% FC for 163 days. We measured light saturated photosynthesis on a mass basis (Asat-mass), stomatal conductance to CO2 on a mass basis (gsCO2-mass), whole-plant water-use efficiency (WUEP), biomass accumulation, specific leaf area (SLA) and total leaf area. At eCO2, Asat-mass increased 28% in well-watered plants and 93% under drought, whereas gsCO2-mass declined 39% in well-watered plants at eCO2, with no effect of drought on gsCO2-mass at eCO2. The total biomass gain improved 73% at eCO2 and over CO2 levels it was reduced (54%) by drought. WUEP improved (188%) at eCO2 in well-watered plants and 262% under drought. SLA declined 23% at eCO2, but the effect of drought on SLA was null. On the contrary, total leaf area was greatly reduced (67%) by drought, but it was not affected by eCO2. The large increase in total biomass and the substantial improvement in WUEP under eCO2, and the sharp decline in leaf area under water stress widen our knowledge on the physiology of this important species for the forest management of large areas in the Amazon region.

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